1. Field of the Invention
The present invention relates generally to large diameter drilling apparatus for drilling large well bores and, more particularly, to circulation systems for transporting to the surface earth cuttings and drilling fluids flowing into the well bore.
2. Description of the Prior Art
Large diameter shafts are basically utilized in the mines for rescue, ventilation, ore and coal hoisting, and general access purposes. In recent years, large diameter shafts have been utilized for emplacing nuclear devices.
Large diameter shafts are usually drilled with rotary flat bottom bits which comprise a main bit body having a plurality of rotatively supported roller cutters attached thereto. The cutters function to contact and disintegrate the earth formation at the bottom of the shaft, thereby creating cuttings or chips at that location.
Various circulation systems utilizing water or drilling mud have been used to flush the earth cuttings away from the shaft bottom. Such circulation systems are usually classified as either direct or reverse circulating systems. In the direct circulation systems, drilling fluid is pumped down the center drill column, across the face of the drill bit at the bottom of the shaft, and up through the annulus of the shaft. In the reverse circulating systems, the drilling fluid is pumped down the shaft annulus to the bottom of the shaft, across the face of the drill bit and up through the center drill column back to the surface. In both systems, when the drilling fluid transporting the chips and cuttings reaches the surface, the fluid is usually pumped through various separating and cleaning devices to separate the cuttings, silt, gas and other materials from the drilling fluid in order to enable the cleaned drilling fluid to be recirculated for further use.
The biggest problem today in drilling large diameter well bores lies in the removal of the drilled cuttings from the bottom of the well bore during the drilling operation, otherwise known as bottom hole cleaning. If the removal of the drilled cuttings is not accomplished efficiently, the cuttings would be reground a large amount before being removed. Regrinding these cuttings to a fine size requires large amounts of energy, decreases the penetration rate and decreases the life of the cutters.
One approach to this problem is described in U.S. Pat. No. 3,360,061. In that patent, a well apparatus is described wherein high pressure gas from gas passages is directed against the bottom of the well bore below the drill to agitate the cuttings and liquids in the well bore. Such nozzles are well known in the art. The problem that still exists with such systems lies in the picking up of the cuttings off bottom to transport it to the surface. The sweep pickups, such as that described in the above-mentioned patent simply do not operate efficiently to alleviate the cutter regrinding problem. Such sweep pickups have inlets allowing for flow from all directions. These opposing flow paths create a stagnation zone at the mid-line of the sweep intake slot close to the base of the bore hole. This stagnation zone prevents the cuttings from being raised from the floor of the bore hole into the flow of the circulation fluid.